Like Nak says, most radios we have in our trucks can listen to wide swaths of spectrum outside the ham bands. Not all radios can do that, but it's pretty common for the past 15 years or so to have wide band RX. Go to VFO mode and punch it in your mic or twirl the tune knob.

That said, many larger police and fire departments have gone to 800MHz and trunked systems. Some (most) are also digital. Like Denver EMS, PD and FD are on EDACS, which is G.E.'s digital trunked system. Being digital our analog radios can only tune the carrier and hear the information, but are unable to decode it back into voice. This is the modem-ish sounds you sometimes hear tuning around. Could also really be just a modem, someone doing digital modes.

Some radios can figure out the protocol and trunking pool map, although the information itself can be encrypted so that it remains scrambled despite being decoded. There are few competing protocols, EDACS, P25, Motorola Type I and Type II, etc. That's not the hard part to figure out if you want to listen to public service. PSK31 (data) and D-STAR (voice) are forms of digital radio, although they're not trunked or anything. Scanners can decode the digital signal and often figure out the squelch tones, but they usually can't descramble the signals if they are encrypted.